The invention relates, in general to a method of intermediate cooling of compressed gases in turbocompressor systems without forming condensate and, more particularly, to an improved method and arrangement of cooling compressed air intermediate successive compressor stages in a multi-stage intercooled compressor system.
To obtain an optimum efficiency, gases are compressed under isothermal conditions. To this end, it is desirable to largely dissipate the compression heat contained in the gas by means of indirect cooling in gas coolers provided between the individual compression stages. Care must be taken to prevent the temperature of the gas in the intermediate coolers from dropping below the dew point, i.e., below the temperature at which the humid gas at the given pressure level is saturated with water vapor.
In the event that the vapor pressure drops below the dew point temperature, the gases taken in along with the air, such as SO.sub.2, SO.sub.3, CO.sub.2 and NH.sub.2, unite with condensed water vapor to form acids and bases which can cause corrosion along their path, on impellers, seals, in the intermediate coolers and the like.
The water condensate droplets entrained with the air stream, moreover, can cause cavitation damage to the impellers of the compressors, which results, for example, in an erosion of the sensitive impeller blades.
The condensate droplets, in addition, can carry dirt particles and corrosion products. Partial evaporation of these droplets during their flow to the next cooling stage leads to the deposition of the dirt particles and corrosion products primarily on the hot walls of the impellers and the distributors. The resulting deposits reduce the cross-sectional area of the flow channels and, thereby, detrimentally affect the performance of the compressor. Dirt deposits on impellers, moreover, may cause strong imbalances which, as is well known, lead to damage during turbo-compressor operations.
To prevent the temperature of the cooled gas from dropping below the dew point, it has been usual to manually control the temperature in the intermediate cooler in accordance with tables, by varying the coolant flow to the cooler.
German patent document No. AS 1 428 047 discloses for example, a control system in which a differential temperature is determined for each compressor stage, computed from the intake temperature of the fluid entering a compression stage and the dew point temperature of this fluid after its exit from the compressor stage. This temperature provides the set point at a continuous control value. However, even such a control system has drawbacks which cannot be overlooked. First, the intermediate cooler temperature cannot be controlled continuously but must be continually adjusted as a function of the variation in time of the intake temperature. Secondly, selection of the differential temperature set point in accordance with the expected maximum intake temperature has the disadvantage that, at a lower actual intake temperature, the adjusted temperature of the intermediate cooler will be too high. This means, that the efficiency of the compressor would be reduced. If, on the other hand, the actual intake temperature exceeds the expected maximum value, the temperature will fall short the dew point temperature, the intake capacity of the compressor will initially be reduced and damage, as noted above, will occur.
Another method of controlling a condensate-free intermediate cooling of compressed gases is known from German patent document No. AS 1 428 033. According to the method disclosed, the temperature of the intermediate cooler is kept above the local dew point temperatures of the gas. This method also has uncontrollable drawbacks. First, extraordinarily large cooling surfaces are needed, since the differential temperature between the gas and the cooling surfaces diminishes as the gas temperature approaches the desired temperature. Second, an adaptation of conventional compressors to this prior art method is extremely difficult if not impossible. Third, at higher cooling water temperatures, the coolers must be operated with purified water, to prevent calcereous deposits in the coolers. Finally, condensate formation cannot always be prevented with this method.
In addition, the two prior art control methods, discussed above, have the disadvantage that the dew point must be determined and introduced into the measuring operation after each cooling stage. With a plurality of coolers, higher static pressures, and higher flow velocities, this becomes considerably expensive.
Another method, disclosed by German patent document No. AS 2 113 038, for example, allows computation of the temperatures in the intermediate coolers of the gas to be compressed, however, since this prior art method measures the intake temperature and is based on a relative humidity of 100%, the computed temperature values are not sufficiently exact to obtain optimum measured values. In addition, in such a method, the fairly considerable effect of the cooler pressure is neglected. As a consequence, at operating pressures below the maximum possible cooler pressure and with relative intake humidities below 100%, the determined temperatures are too high to a considerable extent. The efficiency of the unit is therefore lower than the possible maximum.